The aspartyl residues in the human tooth enamal and dentine and in the eye lens nucleus show increased racemization with age. Based on these results, we predict that the aspartyl residues in any metabolically stable protein will undergo racemization. We are now in the process of determining the extent of aspartic acid racemization in other slowly turned-over human proteins, such as collagen, elastin, myelin sheath proteolipids in order to test this prediction. We are also investigating whether there is any relationship between the racemization in metabolically stable proteins and aging. Since racemization in proteins could affect the functionality of the proteins, it could be directly related to aging in mammals. To test this racemization-aging hypothesis, we propose to use the human eye lens system as a model. Studies have demonstrated that structural changes take place in lens proteins during aging, and we are attempting to determine whether any of the changes are the results of racemization. The extent of racemization in mammalian teeth can be used as a measure of the animal's chronological age. Human teeth from Nepal will be analyzed in order to determine whether racial, environmental, and dietary differences have an effect on the extent of racemization in teeth. Ultimately, it should be possible to use the racemization method to test claims of unusual longevity in the human populations in Ecuador, Hunza, and USSR and to estimate the age distribution of natural mammal populations.